Composite materials including microfibers of cellulose are being investigated enthusiastically in recent years. It is known that cellulose brings about a low coefficient of linear expansion, high modulus of elasticity, and high strength because of the extended-chain crystals thereof. Furthermore, composite materials obtained by compositing microfined cellulose with a resin are attracting attention as materials which show high transparency.
Examples of applications of such composite materials of cellulose fibers (cellulose-fiber composite materials) having high transparency and a low coefficient of linear expansion include transparent substrate materials for electrical/electronic devices represented by flat-panel displays, OLED illuminators, solar cell panels, etc. However, there are cases where in steps for producing such devices, the transparent substrate materials are required to undergo a heat treatment. For use in these applications, materials which take a color upon the heat treatment are undesirable.
Namely, in order for those cellulose-fiber composite materials to be utilized industrially, all of the following requirements must be satisfied:
the cellulose fibers to be used have excellent suitability for fibrillation;
the cellulose fibers, when composited with a resin, give a composite material which is capable of retaining high transparency; and
the cellulose-fiber composite material does not take a color even when subjected to a heat treatment.
Patent document 1 discloses a process for producing microfibers of cellulose by subjecting raw cellulose fibers constituted of purified wood flour to a fibrillation treatment with a high-pressure homogenizer. Patent document 2 discloses a process for producing microfibers of cellulose by conducting a fibrillation treatment using an ultrasonic propagator.
Methods for enhancing fibrillation by using a chemical treatment in combination with any of those fibrillation techniques based on a physical treatment have also been reported. For example, patent document 3 discloses a technique for providing cellulose fibers having enhanced dispersibility in water and excellent suitability for fibrillation, by introducing carboxy groups into cellulose fibers by utilizing an oxidation reaction of the surface of cellulose with an N-oxyl compound. This method, however, is undesirable because the production cost is high since the N-oxyl compound to be used for the cellulose oxidation is generally expensive, the oxidation reaction necessitates many kinds of chemicals, and the production steps are complicated.
Patent document 4 discloses a method for enhancing suitability for fibrillation by conducting a chemical treatment in which cellulose fibers are treated with ozone. The ozone treatment of cellulose has hitherto been used as bleaching in pulp production, and is one of the inexpensive methods of industrial oxidation.
Patent document 5 discloses a technique for obtaining a composite material which can be inhibited from taking a color upon heating, by using nonwoven cellulose fabric in which the hydroxyl groups have been chemically modified.